The present invention relates to a transmission line junction ferrite circulator for high frequency, high power application, having a Y-junction for three transmission lines and containing one or more spaced apart metal plates in the Y-junction covered with ferrite material and an external magnet producing a magnetic field within the Y-junction that is oriented perpendicular to the ferrite covering. More particularly, the present invention provides a method and means of maintaining the temperature of the ferrite material within a predetermined range even while the circulator operates in a variable ambient environment at high power, on the order of 300 kW and at an operating frequency of 500 MHz.
The Y-junction three port circulator is a well known device. Its usual function is to feed high frequency signals entering any one of the three ports to only one of the other two ports with no reciprocity between ports. This function of the circulator depends upon the ferrite material contained in the Y-junction. When the ferrite material is magnetized by an external magnet, it becomes resonant to electro-magnetic waves of a particular frequency and that resonance gives rise to the non-reciprocal flow of signals through the junction that are at the resonant frequency.
Usually, optimum performance of the circulator is achieved by magnetizing the ferrite to saturation magnetization, because saturation magnetization tends to realize the greatest isolation between two isolated ports and the minimum insertion loss between two coupled ports.
The ferrite saturation magnetization is temperature dependent. A change in saturation magnetization can be compensated for by changing the external magnetic field and for that purpose the external magnetic field may be provided by a permanent magnet and an additional electro-magnet and so the magnetization can be changed as necessary by varying the current in the electro-magnet. In this way, a change in the temperature of the ferrite producing a change in the ferrite saturation magnetization can be compensated for to bring performance to optimum. When the saturation magnetization changes due to a temperature change, it is said that the circulator is destabilized or detuned and when compensation is made by changing the electro-magnet current, the circulator is said to be stabilized or tuned.
Stabilizing or tuning a circulator using an electromagnet as part of the external magnetic field system requires a large electro-magnet in addition to a permanent magnet located outside of the Y-junction. This requirement increases the total size and weight of the circulator. A high power circulator incurs significant heating of the ferrite material and so must be tuned over a relatively wide range of saturation magnetization and this requires large structural size and weight to accommodate the larger magnet system.
Heretofore, the ferrite material in a high power circulator has been cooled using a liquid coolant system with fluid passages inside the Y-junction adjacent the ferrite material therein. A high power waveguide Y-junction circulator of this type is described in U.S. Pat. No. 4,717,895, entitled High Frequency, High Power Waveguide Junction Circulator, which issued Jan. 5, 1988 to Erich Pivit, et al. That patient describes a Y-junction of waveguides with one or more thin metal discs in the junction, each covered on both sides with a layer of ferrite material. The metal discs are oriented at the junction perpendicular to the electric field of waves propagating through the junction and so they do not cause excessive reflections. Heat produced in the layers of ferrite material is carried away by the metal discs and carried from the metal discs to an external reservoir or heat sink for the coolant fluid. For this purpose, a coolant fluid tube or pipe is contained within the metal disc.
The fluid cooled circulator described in the above mentioned patent is intended to carry heat from the ferrite inside the Y-junction to the coolant reservoir outside the junction Clearly, this sort of operation intends that the ferrite temperature be stabilized at a value above the coolant reservoir temperature and above ambient temperature. For example, for the high power waveguide Y-junction circulator described in the above mentioned patent with a liquid coolant system, the desired temperature of the ferrite might be 30.degree. C. and the coolant reservoir temperature might be 25.+-.5.degree. C. Coolant flow would be adjusted to maintain a fixed coolant differential temperature calculated to maintain the ferrite at the desired 30.degree. C. and so an increase in high frequency power through the circulator would require an increase in the coolant flow. With this coolant system, problems can arise when cooling is excessive, which may occur when RF power is reduced or when the coolant reservoir temperature is lower than usual as when the ambient temperature may be lower than usual. Also, the coolant system is not effective to stabilize the ferrite temperature when the ferrite temperature is below the desired value.